IMS is an architectural framework for delivering Internet Protocol (IP) multimedia to mobile users. IMS is designed to deliver integrated multimedia services and create an open, standard-based network. IMS Centralized Services is an approach to the provision of communication services wherein services and service control are based on IMS mechanisms and enablers, and support is provided for a diversity of access networks (including circuit switched and IP based, wireless and wireline). Service continuity between domains, e.g., the circuit switch (CS) domain and the packet switch (PS) domain, is also provided.
Single Radio (SR)-Voice Call Continuity (VCC) is a key aspect of the IMS, allowing a user equipment (UE) to move between different network access technologies while maintaining a consistent user experience. For example, a UE may roam between a 4G PS domain (e.g., IMS) and a 2G/3G CS domain while maintaining call continuity and a consistent user interface. During a standard normal (non-emergency) session in the IMS, the application control of the session is controlled by the home network, e.g., the home Call Session Control Function (CSCF) and Service Centralization Continuity Application Server (SCC AS). This use of the home network allows standard normal sessions to be handed over between a 4G system and a 2G/3G system seamlessly.
Emergency calls, however, are handled differently. In particular, emergency calls are handled by the visiting network such that the local network is the serving network. Emergency calls are routed to a Public Safety Answering Point (PSAP) associated with the serving network and, hence, the current location of the UE. In this case, the home network does not control the call.
The use of the visiting network to handle emergency calls precludes the normal handover procedures from being used. Normal, non-emergency session transfers were controlled by the home SCC AS. Because emergency sessions are not controlled by the home SCC AS, the normal, non-emergency session transfer procedures are not sufficient. As a result, there is a need for a system and a method for providing continuity of emergency sessions for mobility between 4G systems and 2G/3G systems.